The recent miniaturization and increased levels of integration of semiconductor integrated circuits has led to a demand for increases in the resolutions of semiconductor exposure devices (called “exposure devices” hereinafter). Accordingly, advances are being made in the reduction of the wavelengths of light emitted from exposure light sources. Typically, gas laser apparatuses are being used as exposure light sources instead of conventional mercury lamps. For example, a KrF excimer laser apparatus that outputs an ultraviolet laser beam at a wavelength of 248 nm and an ArF excimer laser apparatus that outputs an ultraviolet laser beam at a wavelength of 193 nm are used as gas laser apparatuses for exposure.